In six separate experiments, 7-2% of purified peripheral blood lymphocytes from normal donors were shown to form rosettes with mouse erythrocytes. Normal T and B lymphocytes were separated according to their membrane properties, by E-rosette formation or fractionation on anti F (ab')2 column. The results obtained in both the separation procedures used were in good agreement: T lymphocytes were never found to form mouse red cell rosettes. On the contrary, enrichment of suspensions in B lymphocytes resulted in an increased percentage of mouse red-cell rosette-forming cells. Under the technical condition used, peroxidase-positive monocytes were not shown to form such rosettes. However, if all the mouse red cell rosette-forming cells were shown to be of B nature, not all the B lymphocytes form rosettes with mouse erythrocytes and it is not unlikely that this marker could be characteristic for a subset of Ig-bearing B lymphocytes.